1. Field of The Invention
The present invention relates to a brake control system which is applied to electronically control automotive brakes, and specifically to an automotive brake control system employing an external brake fluid pressure source for optimally controlling braking for(-e applied to vehicle wheels, irrespective of depressing of a brake pedal.
2. Description of The Prior Disclosure
Recently, there have been proposed and developed various automotive brake control systems associated with at least one of advantageous braking force control systems, namely a traction control system generally abbreviated as a TCS which is provided for suppressing excessive driving force exerted on driven wheels during quick depressing operation of the accelerator pedal, such as quick starting, quick acceleration, or the like, an anti-skid brake control system generally abbreviated as an ABS which is provided for preventing brakes from locking vehicle wheels during quick braking or during braking on a low frictional road surface so as to provide maximum effective braking, and a servo-assisted brake control device which is provided for generating a wheel-cylinder pressure higher than a master-cylinder pressure at a relatively light brake pedal pressure. Such conventional brake control systems have been disclosed in a Japanese First Publication Tokkai (Showa) 62-149543 and a SAE document No. 840468 (Page 3, FIG. 3 indicating a simplified functional diagram with regard to only one controlled wheel) published by a well-known Society of Automotive Engineers.
An automotive brake control system of the former prior art, includes two different fluid pressure control valves disposed between a master-cylinder and a wheel-cylinder, namely a first fluid pressure control valve provided for generating a reasonably increased hydraulic pre&sure higher than a master-cylinder pressure based on the depressing amount of the brake pedal, while receiving both the master-cylinder pressure and a preset external pressure created by an external hydraulic pressure source, and a second pressure control valve provided for generating the sum of the reasonably increased hydraulic pressure fed from the first pressure control valve and the master-cylinder pressure, as a wheel-cylinder pressure. The former prior art can provide the servo-assisted brake control by multiplying the master-cylinder pressure with a designated multiplication factor by means of the first pressure control valve and the traction control by applying a controlled wheel-cylinder pressure to the vehicle wheels through the external hydraulic pressure source even when the brake pedal is not depressed, but not provide the anti-skid brake control since the wheel-cylinder pressure cannot be reduced lower than the master-cylinder pressure, i.e., the wheel-cylinder pressure is held at the master-cylinder pressure value at a minimum.
Alternatively, an automotive brake control system of the latter prior art, includes a ball valve disposed between a master-cylinder and a wheel-cylinder, a pressure-reducing unit associated with the ball valve for applying the master-cylinder pressure through the ball valve to the wheel-cylinder or for reducing the wheel-cylinder pressure to a desirable pressure lower than the master-cylinder pressure through its sliding movement of a stepped piston operably employed in the pressure reducing unit, and a three-position electromagnetic solenoid valve connected to an external hydraulic pressure source for outputting a designated control pressure to the pressure reducing unit so as to close the ball valve and thus prevent application of the master-cylinder pressure to the wheel-cylinder. In brief, the pressure reducing unit serves to apply the master-cylinder pressure to the wheel-cylinder in the absence of the control pressure, and to reduce the wheel-cylinder pressure in the presence of the control pressure. The latter prior art can provide the traction control because of application of external hydraulic pressure created by the external hydraulic pressure source to vehicle wheels, irrespective of depression of the brake pedal, and also provide the anti-skid brake control by increasing or decreasing the wheel-cylinder pressure by means of the pressure reducing unit. However, the latter prior art cannot provide the servo-assisted brake control, since the wheel-cylinder pressure cannot be increased higher than the master-cylinder pressure, i.e., the wheel-cylinder pressure is held at the master-cylinder pressure at a maximum.
Alternatively, Japanese Patent First Publication (Tokkai) Heisei 4-87867, which is assigned by the same assignee as the present invention, discloses an improved construction of an automotive brake control system so as to solve the above noted various problems. The improved construction of the brake control system of the prior art includes an electronically controlled hydraulic pressure control valve through which the previously noted three advantageous braking force controls can be reliably achieved. The hydraulic pressure control valve employs a spool slidably enclosed therein and an electromagnetic solenoid provided in one side of the valve for creating one axial sliding movement of the spool, and a pressure chamber provided in the other side of the valve for creating the opposing axial movement of the spool via a master-cylinder pressure directly introduced into the pressure chamber. In this conventional construction, since one directional spool pushing force created by the solenoid is designed to match the opposing directional spool pushing force created by the master-cylinder pressure, the solenoid is set to a relatively large size depending on a maximum master-cylinder pressure created by the master-cylinder. This results in a relatively large size of the entire brake control system.